1. Field of the Invention
Embodiments described herein relate generally to a cathode selection method, and for example, relate to a selection method of a cathode of a beam source used in a charged particle beam lithography apparatus.
2. Related Art
In an electron beam apparatus, an electron gun assembly, which serves as a beam source, is used. There are various devices among the electron beam apparatuses such as an electron beam lithography apparatus and an electron microscope. With respect to electron beam writing, for example, it essentially has an excellent resolution and is used in a production of a precise original pattern.
A lithography technique, which takes a part of the development of miniaturization of semiconductor devices, is an only process in semiconductor manufacturing processes in which a pattern is generated and is very important. In recent years, with advancement in integration density of an LSI, a circuit line width required for a semiconductor device is miniaturized year by year. In order to form a desired circuit pattern on such a semiconductor device, a precise original pattern (also referred to as a reticle or a mask) is required. An electron beam (EB) lithography apparatus is used in the production of such a precise original pattern.
FIG. 18 is a conceptual diagram illustrating an operation of a variable-shaped electron beam lithography apparatus. The variable-shaped electron beam lithography apparatus operates as below. A rectangular opening 411 to shape an electron beam 330 is formed in a first aperture plate 410. A variable-shaped opening 421 to shape the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired rectangular shape is formed in a second aperture plate 420. The electron beam 330 radiated from a charged particle source 430 and having passed through the opening 411 of the first aperture plate 410 is deflected by a deflector, passes through a part of the variable-shaped opening 421 of the second aperture plate 420, and irradiates a target object 340 placed on a stage continuously moving in one predetermined direction (for example, an X direction). In other words, an rectangular shape that can pass through both the opening 411 of the first aperture plate 410 and the variable-shaped opening 421 of the second aperture plate 420 is written on a write region of the target object 340 placed on the stage continuously moving in the X direction. A method in which an arbitrary shape is formed by allowing an electron beam 330 to pass through both the opening 411 of the first aperture plate 410 and the variable-shaped opening 421 of the second aperture plate 420 is called the variable-shaped beam method (VSB method).
In the electron beam writing, along with the miniaturization of integrated circuit, a shot size is decreasing while the number of shots is increasing. As a result, a writing time also becomes longer. Therefore, a reduction of the writing time, or in other words, an improvement of a throughput of the lithography apparatus is desired. In order to improve the throughput of the lithography apparatus, it is necessary to increase a current density of an electron beam. In order to increase the current density, it is necessary to make brightness higher for a cathode of an electron gun assembly, which serves as a beam source. For example, a lanthanum hexaboride (LaB6) crystal is used as the cathode (as disclosed for example in JP-A-2005-228741). In order to increase brightness of a thermionic emission cathode, there is a method of increasing a temperature of the cathode. However, if the temperature of the cathode is increased, a cathode life becomes shorter as an evaporation rate of a cathode material becomes larger. For example, in a cathode using the lanthanum hexaboride (LaB6) crystal as the material, it is difficult to raise the temperature of the cathode, for example, significantly higher than 1800 Kelvin (K). Therefore, there is a limit in achieving the high brightness by increasing the temperature of the cathode to be used.
On the other hand, in a LaB6 crystal, for example, manufactured by a zone melting method and the like, a composition ratio of lanthanum (La) and boron (B), an impurity density, and the like are different depending on a position within the crystal. Therefore, even in a case where a plurality of cathodes is manufactured from the same mass of crystal, the brightness obtained may vary for each completed cathode. Accordingly, even in the case where the plurality of cathodes is manufactured, there is a problem in that there are many cathodes with which the desired value of brightness cannot be obtained when used in electron beam apparatuses.